Thin film deposition techniques are widely used in the manufacturing of microfeatures to form a coating on a workpiece that closely conforms to the surface topography. The size of the individual components in the workpiece is constantly decreasing, and the number of layers in the workpiece is increasing. As a result, both the density of components and the aspect ratios of depressions (i.e., the ratio of the depth to the size of the opening) are increasing. The size of workpieces is also increasing to provide more real estate for forming more dies (i.e., chips) on a single workpiece. Many fabricators, for example, are transitioning from 200 mm to 300 mm workpieces, and even larger workpieces will likely be used in the future. Thin film deposition techniques accordingly strive to produce highly uniform conformal layers that cover the sidewalls, bottoms, and corners in deep depressions that have very small openings.
One widely used thin film deposition technique is Chemical Vapor Deposition (CVD). In a CVD system, one or more precursors capable of reacting to form a solid film are mixed while in a gaseous or vaporous state, and then the precursor mixture is presented to the surface of the workpiece. The surface of the workpiece catalyzes the reaction between the precursors to form a solid film at the workpiece surface. To catalyze the reaction, the workpiece is typically heated to a desired temperature.
Although CVD techniques are useful in many applications, they also have several drawbacks. For example, if the precursors are not highly reactive, then a high workpiece temperature is needed to achieve a reasonable deposition rate. Such high temperatures are not typically desirable because heating the workpiece can be detrimental to the structures and other materials already formed on the workpiece. Implanted or doped materials, for example, can migrate within the silicon substrate at higher temperatures. On the other hand, if more reactive precursors are used so that the workpiece temperature can be lower, then reactions may occur prematurely in the gas phase before reaching the substrate. This is undesirable because the film quality and uniformity may suffer, and also because it limits the types of precursors that can be used.
Atomic Layer Deposition (ALD) is another thin film deposition technique. FIGS. 1A and 1B schematically illustrate the basic operation of ALD processes. Referring to FIG. 1A, a layer or partial layer of gas molecules Ax coats the surface of a workpiece W. The layer of Ax molecules is formed by exposing the workpiece W to a precursor gas containing Ax molecules and then purging the chamber with a purge gas to remove excess Ax molecules. This process can form a monolayer or partial monolayer of Ax molecules on the surface of the workpiece W because the Ax molecules at the surface are held in place by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. Referring to FIG. 1B, the layer of Ax molecules is then exposed to another precursor gas containing By molecules. The Ax molecules react with the By molecules to form an extremely thin layer of solid material on the workpiece W. The chamber is then purged again with a purge gas to remove excess By molecules.
FIG. 2 illustrates the stages of one cycle for forming a thin film using ALD techniques. A typical cycle includes (a) exposing the workpiece to the first precursor Ax, (b) purging excess Ax molecules, (c) exposing the workpiece to the second precursor By, and then (d) purging excess By molecules. In actual processing, several cycles are repeated to build a thin film having the desired thickness. For example, each cycle may form a layer or partial layer having a thickness of approximately 0.1-1.0 Å, and thus several cycles are required to form a layer having a thickness of approximately 60 Å.
FIG. 3 schematically illustrates a single-wafer ALD reactor 10 having a reaction chamber 20 coupled to a gas supply 30 and a vacuum 40. The reactor 10 also includes a workpiece holder 50 that supports the workpiece W and a gas dispenser 60 in the reaction chamber 20. The gas dispenser 60 includes a plenum 62 operably coupled to the gas supply 30 and a distributor plate 70 having a plurality of holes 72. In operation, the workpiece holder 50 heats the workpiece W to a desired temperature, and the gas supply 30 selectively injects the first precursor Ax, the purge gas, and the second precursor By, as shown above in FIG. 2. The vacuum 40 maintains a negative pressure in the chamber to draw the gases from the gas dispenser 60 across the workpiece W and then through an outlet of the reaction chamber 20.
One drawback of ALD processing is that it has a relatively low throughput compared to CVD techniques. For example, each Ax-purge-By-purge cycle can take several seconds. This results in a total process time of several minutes to form a 60 Å thick layer. In contrast to ALD processing, CVD techniques require only about one minute to form a 60 Å thick layer. The low throughput of existing ALD techniques limits the utility of the technology in its current state because it may be a bottleneck in the overall manufacturing process.
Another drawback of existing ALD reactors is that the purge pulses may not remove all of the Ax or By molecules from the reactor. As a result, during the pulse of the reactive gas By, remaining molecules of the reactive gas Ax within the reactor volume will react with the By molecules and produce unwanted particles in the chamber. This may cause defects and/or non-uniformities on the workpiece.